How should I ask about that message I saw on my husband's phone?

Hey girls, hi! I want to ask something. My husband and I are actually fine, but lately, there's been some distance between us. Last night, I saw his phone left open on the table for a moment, and I noticed a conversation with a female coworker that isn't really work-related. It's not exactly flirty, but there's something weird about it. I'm hesitant to ask because I'm worried it might disrupt our peace, but it's really eating away at me inside. What do you think I should do? Please give me some advice.